Conventionally the temperature control device of this type has been used for temperature control of an injection molding machine which is equipped with a heating cylinder and a cooling jacket around the cylinder.
Referring to FIG. 6 one example will be described hereinafter.
In FIG. 6, a heating cylinder 10 comprises a screw 12 which is capable of reciprocating and rotating motion. A heater 14 of a heating means and a cooling jacket 16 of a cooling means are provided on the peripheral surface of the heating cylinder 10. A thermocouple 18 for temperature detection is disposed at each temperature control zone where the heater 14 and the cooling jacket 16 are provided. The heater 14 is connected through a switch 22 to a power supply 20. The cooling jacket 16 is connected through an electromagnetic valve 26 to a cooling medium supply source 24 such as a blower for supplying cooling air.
The switch 22 and the electromagnetic valve 26 are respectively connected to a temperature controller 28.
The temperature controller 28 comprises a temperature control circuit 30 for setting proportional bands which determine heating and cooling energies for heating and cooling medium in the heater 14 and the cooling jacket 16, respectively. The temperature controller may include other necessary means such as arithmetic processing units.
The term "proportional band" will be briefly described hereinafter in connection with a typical example in which a controlled object and a control unit constitute a feedback system.
When the temperature of a heavy oil as a controlled medium is controlled to a constant value by means of a heater in the feedback system, the control unit consists of a detection means for detecting oil temperature, a control means for comparing the oil temperature with a desired value by means of a controller and generating an appropriate signal, and an operation means for equalizing the oil temperature to the desired value.
In this case, a feedback circuit is formed between the controlled medium (heater) and the control unit.
Then, in the detection means, the thermocouple, the heavy oil and the oil temperature act as a primary element, a controlled medium and a controlled variable, respectively. The control means may consist of a power unit and a final control element, in which vapor and vapor flow rate are regarded as a control agent and a manipulated variable, respectively.
In such a feedback system, a proportional positioning action which is generally referred to as P-action can be determined by the following equations: ##EQU1## where x=controlled variable; v=desired value; .mu..sub.v =position of final control element. Namely, it is noted that the controlled variable is in proportion to a position of the final controlled element (manipulated variable).
In the above equations, K.sub.p is referred to as a proportional sensitivity or proportional gain and S which is represented as of an inverse of K.sub.p is referred to as proportional band.
Namely, the proportional band is a percentage representation of a range of the controlled variable in which the final control element is controlled over all manipulated variable range in proportional relation, relative to all scale range of a controller. For example, when the scale range of the controller is 0.degree. C. to 200.degree. C. and the temperature as a controlled variable varies by 40.degree. C. in response to all manipulated variable of a valve as a final control element, the proportional band is 20%.
FIG. 7 shows such a relation between controlled variable, manipulated variable and proportional band.
Even though any load change may occur in the P-action, there is no change in relation between the controlled variable and the position of the final control element.
Namely, only one position of control valve corresponds to a given recording pen position of the controller.
Accordingly, the recording pen position representing the controlled variable corresponds to a desired position, i.e., a set point only when a special load is given. Generally, the recording pen is allowed to be deviated from the set point even under balanced condition. The deviation is referred to as residual deviation or offset.
A general proportional positioning type controller has an adjusting mechanism referred to as a manual reset. The adjusting mechanism acts such that the pen position corresponds to the set point under balanced condition by moving the proportional band shown in a graph of FIG. 5(a) in the direction of the abscissas according to changes in process condition as illustrated in FIG. 5(b).
For example, using a controller having a scale mark of 0.degree. C. to 200.degree. C. in which the proportional band is 20% (namely 40.degree. C. range), an oil temperature controlled to a set point of 150.degree. C. whereupon an opening of a valve through which a vapor is passed is 50% and the system is under balanced condition. In such a case, if a flow rate of oil is doubled, the valve must be opened until its opening reaches 75% in order to obtain a necessary quantity of heat. In this case, the 75% valve opening causes the recording pen to be moved down to indicate 140.degree. C. In order to obtain the 75% valve open rate and eliminate an offset without temperature drop, the manual reset mechanism moves the proportional band to the right such that the valve opening increases from 50% to 75%.
As the proportional band becomes narrow, namely, as proportional sensitivity becomes increased, control operation is close to on-off operation. To this end, the offset is increased while a cycling is reduced.
Therefore, P-action is rendered correctable in size and safe in operation by controlling the proportional band and safe in operation. Generally, it can be used for not only a process which has slow load fluctuation and small or medium reaction rate, but also an astatic process such as on-off operation.
Further, FIG. 7 shows a typical energy balance in which a temperature is controlled by controlling heating and cooling energy supplies.
Namely, heating or cooling energy is varied from 0% to 100% in accordance with predetermined control period and deviation of an actual temperature from desired set temperature.
In this case, energy supply by heating means and energy supply by cooling means are often not equal for controlling a heat quantity of the controlled medium which is subjected to temperature control. In an injection molding machine illustrated in FIG. 6, energy supply by heating means is larger than that by cooling means. As shown in FIG. 7, heating energy and cooling energy which are supplied when the actual temperature is deviated from the desired set temperature, are respectively represented as H and C.
The H and C are varied depending upon heating and cooling capacity of the injection molding machine. Also, the energies are varied under such conditions as amounts of internal calorific power and of heat radiation to the external caused due to shearing action of a screw upon resin's plasticization and melting.
In FIG. 7, 100% cooling energy C.sub.1 is supplied at the temperature (t+d).degree. C., where the desired set temperature is t.degree. C. In temperature range from (t-d).degree. C. to (t+d).degree. C., heating energy and cooling energy are respectively supplied in different amounts.
For example, at temperature (t-e).degree. C., heating energy H.sub.2 and cooling energy C.sub.2 are respectively supplied in different amounts.
Therefore, respective supplies of heating energy and cooling energy are kept to be balanced at temperature (t+f).degree. C. at which heating energy H.sub.3 and cooling energy C.sub.3 are equal to each other.
In this case, an offset of f.degree. C. is generated at proportional action control. In order to eliminate the offset, Pi control which is a combination of proportional control and integral control can be used.
In a controlled object having both heating means and cooling means, temperature controls for heating means and cooling means are performed by using one temperature control circuit.
In such controls as P (proportional)-action control, Pi (proportional plus integral)-action control, PiD (proportional plus integral plus derivative)-action control and the like, the ratio of energy supply period to predetermined control period is varied on the basis of controlled variable to be calculated from size or change velocity of the deviation of the actual temperature of the controlled medium from the desired set temperature. Therefore, both control constant such as P, i, D and the proportional band for control are identically controlled.
However, a conventional temperature control device has the following disadvantages.
Namely, dispite that the aforementioned offset can be eliminated by the Pi-action control, a hunting effect over a large temperature range relative to the desired temperature t.degree. C. is generated, as shown in FIG. 4, due to heating energy H.sub.3 and cooling energy C.sub.3 supplied in different amounts during constant control period.
Further, in the controlled medium having both heating means and cooling means, there is often a considerable difference in volume between heating load, i.e., a temperature-rise characteristic due to heating energy supplied by heating means, and cooling load, i.e., a temperature-drop characteristic due to cooling energy supplied by cooling means. For this reason, a stable temperature control can not be easily performed by using a common temperature control circuit.